The present invention relates to a coreless motor having a permanent magnet fixed to a casing and formed with an even number of poles, a rotor coil unit integrated with an output shaft, and a commutator.
There has been known the conventional coreless motor of the cylindrical type used in office equipment, a robot, a medical instrument and so on, which require a high performance motor featuring a high efficiency as well as a quick start/stop motion.
FIG. 7 is a structural diagram of the prior art motor. In manner similar to other types of motors, there are included a rotor 5 and a stator composed of a permanent magnet 4. The rotor 5 includes a rotor coil unit 7 which solely contributes to generation of output torque. The motor further comprises a rotor holder 6 which supports the coil unit 7, a commutator mechanism composed of commutator segments 8 for controlling a rotation direction, and an output shaft 9 for rotatably supporting the rotor. On the other hand, the stator is constructed such that a bearing housing 2 having bearings 3 contains therein the fixed magnet 4. A casing 1 is fixed to the bearing housing 2 to cover entirely the housing, while the casing 1 functions as a return yoke of the magnet 4. An electric current is fed to the rotor Coil unit 7 of the rotor 5 via lead wires 12, contacts of a brush 10 and respective commutator segments 8. A pair of washers 13 are disposed on outer faces of the respective bearings 3, and a stopper ring 14 is fixed to the output shaft 9 in order to suppress axial movement of the rotor 5 during the course of rotation. A cap 15 is fixed to an inner portion of a rear cover 11 to block entry of exterior dust.
The conventional coreless motor has a winding structure shown in FIGS. 8, 9 and 10 wherein the rotor coil unit 7 has a plurality of coils terminating in terminals or taps p, q, r and s. When the coils are laid out as shown in FIGS. 9-10, they have a diameter dimension La. The rotor coil unit 7 of the coreless motor is connected electrically as shown in FIG. 11 to form the rotor 5 as shown in FIG. 13. As understood from FIG. 13, this coreless winding is formed such that the coil unit 7 of the rotor 5 has a radial thickness defined by two layers of the windings. However, since the radial thickness is limited to twice the diameter of the coil wire, the conventional structure has the drawback that the coil wires cannot be wound thick freely thereby limiting the amount of copper in the coil unit.
Particularly in reducing the motor size, while an energy product of the magnet has been improved efficiently, a magnetic motive force of the coil of the rotor has not been improved efficiently. Stated otherwise, in reducing the motor size, the magnetic loading has been improved while the electric loading has not been improved. The motor output torque cannot be optimally improved unless a design balance is ensured with respect to a ratio between the magnetic loading and the electric loading. In view of this, it is necessary to broaden optimally a space gap between the magnet and the casing so as to increase the copper amount of the rotor coil unit. In order to increase the copper amount of the coreless coil, it is necessary to increase the radial thickness of the cylindrical coil unit.
It might be advisable to form multiple stages of the cylindrical coil units. However, for example, in case that respective stages of the coil units are connected in parallel to each other as shown in FIG. 12, there may be caused the drawback that an inductive voltage coefficient Ke cannot be raised adequately in the multiple-stage motor. There is a problem that the series connection is needed in order to increase the value Ke in the prior art.